FIG. 1 is a highly simplified view of a digitizing pad 3 and its associated stylus 6. There are four current-to-voltage amplifiers (labeled I-to-V), one at each comer. The stylus 6, when activated, produces a signal 9, as indicated in FIG. 2. This signal induces currents I1-I4, which are detected by the I-to.sub.-- V amplifiers.
The I-to-V amplifiers each produce a voltage indicative of the size its respective current. Processing circuitry, known in the art, receives the voltage signals, and computes the position of the stylus signal 6, based on the voltage signals.
The currents I1-I4 are induced because the stylus 6 acts as one plate of a capacitor. The digitizing pad (which bears a resistive surface of a material such as indium tin oxide) acts as the other plate. As FIG. 3 indicates, when negative charge is applied to the tip 2 of the stylus 6, a positive charge is induced on the surface of the pad 6. Currents I1-I4 supply this positive charge.
Conversely, as in FIG. 4, a positive charge on the tip 2 of the stylus 6 induces a negative charge on the pad. The currents I1-I4 supply this negative charge.
The relative magnitudes of the currents depend on the position of the stylus. When the stylus is located as shown in FIG. 5, current I1 is the largest, current 14 is next largest, and currents I2 and I3 are nearly the same.
When the stylus is located as shown in FIG. 6, current I4 is the largest, current I1 is next, and so on. The differences in current magnitudes cause differences in the voltages produced by the I-to-V amplifiers of FIG. 2.